This invention relates to compound archery bows and, in particular, to an anchor for the cable in a compound bow.
Compound bows differ from long bows in that a block and tackle mechanism is used to bend the bow: pulleys or wheels are attached at the free ends of the limbs to obtain a mechanical advantage in bending the bow. The limbs can be made approximately three times as stiff as for a longbow. Eccentrically mounted wheels enable one to use a much higher maximum draw weight because they provide a substantial "let off" or reduction in the holding force of a drawn bow. The combination of stiffer limbs and greater draw weight applies large forces to the components of a compound bow.
Compound bows typically use two wheels or pulleys, with lacing wound from an anchor at one end of a first limb to the pulley at the end of the other limb, to the pulley at the first end, then back to an anchor at the other end. The middle span between the wheels contains the serving or region for receiving the nock of the arrow. As used herein, "lacing" can refer to a single piece bowstring or a three piece line comprising two end cables connected by a central stretch between the wheels which forms the bowstring. The serving is the central portion of the bowstring.
A compound bow typically has a three piece lacing. Each cable is fastened at one end to the free end of the bow limb and spans the length of the bow to wrap around the wheel at the other end of the bow. The cable exits the wheel and attaches to the bowstring by what is known as a teardrop connector. In some bows, the bowstring and cables are interconnected by the wheels.
For two wheel compound bows, the cables are anchored, i.e. have their free ends attached, to the end of the limb. In some cases, the end of the cable is simply looped over the axle upon which the wheels rotate. In other cases, a simple fitting connects the cable end to the axle upon which the wheels rotate. In still other cases, a U-shaped bracket has its arms attached to each end of the axle upon which the wheels rotate. The base of the U is attached to the end of the cable. The U-shaped bracket is typically made from metal. U.S. Pat. No. 4,300,521 discloses a bracket in the form of a short length of cable in a loop wound half way around an idler pulley. The ends of the short cable are attached to the axle. The end of the cable from the lacing is attached to the idler pulley. In all cases, the anchor is placed arbitrarily, i.e. without regard to the forces on the end of the limb.
The need arises not only to attach the ends of the cables to the limbs but also to avoid having the cables and bowstring rub and wear each other; such rubbing could result in inconsistent operation of the bow. Thus, the cables and string must be spaced slightly so that they are not coplanar. The wheels or pulleys at the ends of the limbs turn on a single axle, to which the anchor for the end of the cable is also attached. Thus, these components are spread out along the axle across the width of the limb.
The substantial forces involved combined with the spacing of the bowstring and cables at the ends of the limbs cause torques on the free ends of the limbs which twist the limbs. To further complicate the matter, the torques vary as the bowstring is drawn and released. In general, the tension on the bowstring is greatest when the bow is at rest and least when the bow is fully drawn. The situation is reversed in the cables: the tension is least when the bow is at rest and greatest when the bow is drawn. As a result of these changing forces, the ends of the limbs twist one way and then the other as the bow is fired such that the axles of the wheels are no longer perpendicular to the bowstring.
The torque on the end of each limb is a net torque, the result of three components: the two cables and the bowstring. Since the wheel receiving a first cable, the wheel receiving the bowstring, and the anchor for the second cable are spread out along the axle, there is a moment arm, i.e. a distance from a fixed reference point, associated with each element. Mathematically, any reference point could be chosen but the longitudinal center of the axle is convenient because it makes it easier to visualize balancing the moments.
The forces (tension) in the cables and the bowstring combined with their respective distances from center are the respective moments of these elements. The net torque from these moments tends to twist the respective limbs; however, the limbs resist twisting. This counteracts the net torque and the limbs stop twisting at some point, e.g. ten degrees of twist. As used herein, "net torque" refers to the resultant torque from the cables and the bowstring, as described above.
In view of the foregoing, it is therefore an object of the invention to provide a means for adjusting the net torque on the ends of the limbs of a compound archery bow.
Another object of the invention is to provide a means for minimizing the net torque on the end of the limbs of a compound archery bow.
A further object of the invention is to provide a means for setting the zero net torque point to any place in the draw range of a compound archery bow.
Another object of the invention is to provide a means for minimizing the average net torque on the ends of the limbs of a compound archery bow as it is drawn and fired.